Added text to state that overdiagnosis occurs when screening procedures detect cancers that would never become clinically significant. Also added that because nearly all cases of cancer and ductal carcinoma in situ (DCIS) will be treated, women with clinically insignificant cancers will suffer treatment-related side effects unnecessarily.

Added text to state that one approach to understanding overdiagnosis is to examine the prevalence of occult cancer in women who died of noncancer causes; in an overview of seven autopsy studies, the median prevalence of occult invasive breast cancer was 1.3% and of DCIS was 8.9% (cited Welch et al. as reference 16 and Black et al. as reference 17).

Added text to state that overdiagnosis can be indirectly measured by comparing breast cancer incidence in screened populations with breast cancer incidence in unscreened populations, and these comparisons can be further complicated by differences in the populations, such as time, geography, health behaviors, and hormone usage. Included text to state that calculations of overdiagnosis can vary in the adjustment for lead-time bias (cited Duffy et al. as reference 18 and Gøtzsche et al. as reference 19). Also added text to state that an overview of 29 studies found calculated rates of overdiagnosis of 0% to 54%, with rates from randomized studies between 11% and 22%. Additionally, in Denmark, where screened and unscreened populations existed concurrently, the rate of overdiagnosis of invasive cancer was calculated to be 14% and 39%, using two different methodologies; however, if DCIS cases were included, the overdiagnosis rates were 24% to 48% (cited Nelson et al. as reference 20 and Jørgensen et al as reference 21).

Added text to state that theoretically, in a given population, the detection of more breast cancers at an early stage should result in a subsequent reduction in the incidence of advanced-stage cancers; thus, the detection of more early-stage cancers through screening probably represents overdiagnosis.